Hitherto, various attempts have been made for utilization of the solar energy reaching the earth's surface. However, most of the prior art solar energy systems involve the transformation of the captured solar energy into electric or thermal energy and, as far as the inventor is aware, there are very few that directly utilize the solar beams themselves as a source of optical energy. It is obvious that, in lighting applications, the use of solar energy in the form of optical energy is advantageous, since transformation loss is obviated. This is possible by collecting the solar beams on the earth's surface where the solar beams arrive, introducing them into an end of a bundle of light guides or conductors for transmission therealong and then allowing them to issue out of the other end of the light guides to illuminate the spaces where illumination is required. The solar beams as introduced into the light conductors propagate therealong by being totally reflected at the boundary surface thereof. The solar beams having an angle of incidence which is larger than the critical angle of incidence of the light guides are not reflected by the boundary surface and, thus, are able to transmit through the boundary surface to leak out of the light guides. The critical angle of incidence is normally not greater than about 24.degree. for a light conductor made from pure silicate. Accordingly, the essential requirement for a solar beam collecting apparatus intended for the above mentioned purposes is that the solar beams must have an angle of incidence as small as possible and not larger than about 24.degree. and they must be converged sufficiently for their introduction into the light guides which often have a small diameter.
The inventor has previously proposed a solar beam collecting and converging apparatus comprising a plurality of lens systems, (Japanese Patent Application Laid-Open Publication No. 55-28055(1980)). However, this apparatus suffers from the disadvantage that due to chromatic aberration the focus of a lens system varies considerably depending on the wave length of incident light rays. This fluctuation of the focal length leads to the facts that those light rays which have a large wave length are not adequately converged towards the inlet of the light conductors while the angle of incidence of those light rays having a small wave length become larger than the critical angle of incidence of the material forming the light guides, whereby the problem of the light leakage arises. As a result, it was not possible to introduce the whole components of a white light, that is, the sun light, which have passed through the marginal area of a converging lens, effectively into the light conductors. If a single lens system having a relatively long focal distance to a lens diameter, that is, a small angle of incidence is to be used, then the overall dimension of the collecting apparatus which needs to have a large light receptive surface area becomes prohibitive.